The glossary

Understanding Double Glazing Jargon

window quote can read like a wall of initials — U-values, WER bands, argon, low-E, spacer bars. None of it is difficult once it is translated. This glossary decodes the terms you will actually meet on a double glazing quote, so the specification stops being a barrier and becomes something you can judge.

A close-up of a printed glazing specification with technical terms — Every one of these terms describes something you are paying for — worth knowing what.

Installers use this language every day, so it is easy to forget that most homeowners meet it once a decade. When a salesperson reels off “A-rated units with a warm-edge spacer and argon fill,” that should tell you something specific — not simply sound impressive. Keep this page open while you read a quote and every line becomes checkable.

Thermal performance

U-value: A measure of how much heat passes through the window, in W/m²K. Lower is better — a lower U-value means less heat escaping. Modern double glazing typically achieves a whole-window U-value around 1.2–1.6, where old single glazing might be near 5.
Window Energy Rating (WER): A colour-coded band from A++ down, similar to the label on a fridge, summarising the whole window’s energy performance. A and above is the common target for new double glazing.
Low-emissivity (low-E) glass: Glass with an invisibly thin metallic coating that reflects heat back into the room while still letting light through. It is one of the biggest contributors to a good U-value.
Argon fill: An inert gas sealed between the panes in place of ordinary air. It conducts heat less readily, improving insulation. Krypton is used occasionally in slimmer units.

According to the Energy Saving Trust, replacing old single glazing with energy-efficient double glazing can reduce heat lost through windows, though the typical saving depends heavily on the home, its heating and what you started with. Treat any figure as an attributed typical range, not a promise.

The glass unit

Sealed unit / IGU: The insulated glass unit — two or three panes bonded together with the gas-filled cavity between them. It is the part that fails if the seal breaks and misting appears.
Spacer bar: The strip around the edge of the unit that holds the panes apart. A “warm-edge” spacer made of composite rather than aluminium reduces heat loss and condensation at the glass edge.
Toughened / laminated glass: Safety glass required in certain locations such as low-level windows and doors. Toughened glass shatters into blunt pieces; laminated glass holds together on a plastic interlayer.
Triple glazing: Three panes and two cavities rather than two panes and one. It can improve insulation and noise reduction, at extra cost and weight — not always worth it for every home.

A cut-away view showing the two panes and sealed cavity of a double-glazed unit — Inside a sealed unit: two panes, a spacer bar and the gas-filled cavity that does the insulating.

Frames and opening styles

uPVC: Unplasticised polyvinyl chloride — the most common frame material in the UK, low-maintenance and cost-effective. Quality varies by profile system and wall thickness.
Casement window: A window hinged at the side or top that swings open. The most widespread modern style.
Sash window: A window that slides vertically, traditional on period and Victorian homes. Modern versions add double glazing while keeping the look.
Tilt-and-turn: A window that can tilt inward from the top for ventilation or swing fully open for cleaning and escape.

White uPVC casement windows on the front of a brick semi-detached house — The style names on your quote describe how each window opens — and how it will look.

Regulations and paperwork

Trickle vent: A small controllable vent built into the frame to provide background ventilation. Current building regulations require them on most replacement windows.
FENSA / Certass: Competent person schemes that let installers self-certify that replacement windows meet building regulations, and register the work. You receive a certificate you will need when you sell the home.
Insurance-backed guarantee: A guarantee underwritten by an insurer so it still stands if the installer stops trading before the term ends.

With the vocabulary decoded, the next move is to see how these terms sit within a real document. Read them in context with our anatomy of a double glazing quote, check nothing is missing against what a window quote should include, and return to the complete window quote guide whenever you need the map. If frame material is still an open question, our ranked verdicts on window materials weigh up the options, and you can compare quotes from different firms or or go direct for a faster quote once you know what to ask for.